Read file into std::vector

Question

I'm trying to read a file in binary format into a std::vector<std::byte>

  std::ifstream fStream(fName, std::ios::binary);

  std::vector<std::byte> file_content((std::istreambuf_iterator<std::byte>(fStream)),
                                        std::istreambuf_iterator<std::byte>());

but I'm getting this error (which to me looks like istreambuf_iterator is missing an overload for std::byte)

error: no matching function for call to ‘std::istreambuf_iterator<std::byte>::istreambuf_iterator(std::ifstream&)’
     std::vector<std::byte> file_content((std::istreambuf_iterator<std::byte>(fStream)),

Am I doing something wrong ? And if yes what is the best way to do this ?

Thanks!

Possible duplicate of https://stackoverflow.com/questions/47481231/what-is-the-purpose-of-stdbyte — Dúthomhas, Feb 24 '18 at 16:32
@PasserBy If I try using `istream_iterator` if get `error: no match for ‘operator>>’ (operand types are ‘std::istream_iterator::istream_type {aka std::basic_istream}’ and ‘std::byte’)` — Théo Champion, Feb 24 '18 at 16:48
@ThéoChampion That is because there is no default `operator>>` that reads a `std::byte` from a `std::istream` — Remy Lebeau, Feb 24 '18 at 19:23

Remy Lebeau · Accepted Answer · 2018-02-24T19:22:09.363

I'm trying to read a file in binary format into a std::vector<std::byte>

You are using std::istream_iterator, which reads from an std::istream using operator>>, which performs a formatted read instead of a binary read by default. Use std::istream::read() to read binary data.

If you want to use std::istring_iterator to read bytes, you would need to define a custom operator>> that calls std::istream::read() or std::stream::get(). But this would be inefficient since it would read 1 byte at a time. It is better to call read() directly to read blocks of multiple bytes at a time. For instance, query the file size, preallocate the std::vector to that size, and then read() from the std::ifstream directly into the std::vector for that size.

Update: I just noticed that you are using std::istreambuf_iterator instead of std::istream_iterator. std::istreambuf_iterator does not use operator>>, so it would be better suited for reading bytes. However, it still reads 1 byte at a time, so what I said about using std::istream::read() to read multiple bytes at a time still applies.

If I understand well, you mean something like https://godbolt.org/g/ezWhp2 but this will not work if I use `std::byte` instead of `char` right ? — Théo Champion, Feb 24 '18 at 17:48
@ThéoChampion yes, something like that, and yes, it works fine for `std::byte`, eg: `file.read((char*)buffer.data(), length);` — Remy Lebeau, Feb 24 '18 at 19:13

score 4 · Answer 2 · answered May 29 '19 at 01:53

4

you should be able to do it like this:

  std::basic_ifstream<std::byte> fStream{fName, std::ios::binary};

  std::vector<std::byte> file_content{ std::istreambuf_iterator<std::byte>(fStream), {} };

answered May 29 '19 at 01:53

randomguy

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What you mentioned above is not working. terminate called after throwing an instance of 'std::bad_cast' what(): std::bad_cast – Ahmed Dec 30 '20 at 10:59

score 0 · Answer 3 · answered Nov 03 '20 at 16:00

0

unfortunately only char iterators are implemented in STL, so

std::istreambuf_iterator<std::byte>

will cause an error. so use

std::istreambuf_iterator<char>

instead.

answered Nov 03 '20 at 16:00

Oleg Kokorin

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Read file into std::vector

3 Answers3